U.S. patent application number 14/406613 was filed with the patent office on 2015-06-25 for vehicle control system and method to provide desired wheel slip.
The applicant listed for this patent is Jaguar Land Rover Limited. Invention is credited to Paul Beever, Michael Blyth.
Application Number | 20150175009 14/406613 |
Document ID | / |
Family ID | 46605738 |
Filed Date | 2015-06-25 |
United States Patent
Application |
20150175009 |
Kind Code |
A1 |
Beever; Paul ; et
al. |
June 25, 2015 |
VEHICLE CONTROL SYSTEM AND METHOD TO PROVIDE DESIRED WHEEL SLIP
Abstract
The invention relates to a control system and methods for
controlling motion of a vehicle over a surface. The control system
limits an amount of drive torque that may be applied to one or more
wheels of the vehicle to prevent an amount of wheel slip from
exceeding a prescribed value. The prescribed value of wheel slip is
determined in dependence at least in part on vehicle speed. In
addition, method claims 28 and 29 are directed to providing drive
torque, independent of vehicle speed, which corresponds to or
exceeds the maximum traction force. These methods are suitable for
highway driving with high friction or off road driving on soft
terrain, respectively.
Inventors: |
Beever; Paul; (Coventry,
GB) ; Blyth; Michael; (Coventry, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Jaguar Land Rover Limited |
Coventry |
|
GB |
|
|
Family ID: |
46605738 |
Appl. No.: |
14/406613 |
Filed: |
June 11, 2013 |
PCT Filed: |
June 11, 2013 |
PCT NO: |
PCT/EP2013/062017 |
371 Date: |
December 9, 2014 |
Current U.S.
Class: |
701/22 |
Current CPC
Class: |
B60W 2540/10 20130101;
B60K 2007/0092 20130101; B60L 15/36 20130101; B60W 2520/26
20130101; B60W 2720/30 20130101; B60W 50/082 20130101; B60L 15/20
20130101; B60K 7/0007 20130101; B60W 2720/406 20130101; Y02T 10/72
20130101; B60W 2720/403 20130101; B60W 30/18172 20130101; B60W
10/08 20130101; B60W 2520/10 20130101; B60K 28/16 20130101; B60W
2050/0012 20130101; B60L 3/106 20130101; B60W 2552/40 20200201;
Y02T 10/7258 20130101; B60W 30/02 20130101; B60W 30/18027 20130101;
B60W 2720/26 20130101 |
International
Class: |
B60L 3/10 20060101
B60L003/10; B60L 15/36 20060101 B60L015/36; B60K 7/00 20060101
B60K007/00; B60L 15/20 20060101 B60L015/20 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 11, 2012 |
GB |
1210282.8 |
Claims
1. A control system that controls motion of a vehicle over a
surface, the control system being operable to limit an amount of
drive torque that may be applied to one or more wheels of the
vehicle that are arranged to be driven to a prescribed torque value
so as to prevent an amount of wheel slip from exceeding a
prescribed value of wheel slip, wherein the prescribed value of
wheel slip is determined in dependence at least in part on vehicle
speed, and wherein the control system is operable to limit a rate
of increase of applied drive torque to a prescribed maximum value
as the value of applied drive torque increases toward the
prescribed torque value.
2. The control system of claim 1, wherein the prescribed value of
wheel slip is determined in further dependence at least in part on
one or more selected from: a coefficient of friction between the
wheel and the surface, and a value of surface drag experienced by
the vehicle.
3. (canceled)
4. The control system of claim 1, wherein the prescribed value of
wheel slip is determined in dependence on one of: vehicle speed
only, vehicle speed and a coefficient of friction between a wheel
and the surface only, or reference to vehicle speed and one or more
other parameters in dependence on a driving mode in which the
control system is operating.
5.-6. (canceled)
7. The control system of claim 1, wherein the prescribed value of
wheel slip is defined substantially as one of: that at which a
maximum amount of tractive force acting to cause movement of the
vehicle may be achieved, and that at which a maximum amount of
tractive force causing movement of the vehicle may be achieved.
8. (canceled)
9. The control system of claim 1, wherein the prescribed value of
wheel slip substantially corresponds to a prescribed proportion of
that at which a maximum amount of tractive force causing movement
of the vehicle may be achieved, and wherein the prescribed
proportion corresponds to a value less than that at which a maximum
amount of tractive force causing movement of the vehicle may be
achieved.
10. The control system of claim 9, wherein the prescribed
proportion substantially corresponds to one selected from amongst
from (i) greater than or equal to 70% but less than 90%, and (ii)
greater than or equal to 90% but less than 100%.
11. The control system of claim 9, wherein the prescribed
proportion corresponds to a value greater than that at which a
maximum amount of tractive force causing movement of the vehicle
may be achieved.
12. The control system of claim 11, wherein the prescribed
proportion substantially corresponds to one selected from amongst
(i) greater than 100% but less than or equal to 110%, (ii) greater
than 110% but less than or equal to 120%, and (iii) greater than
120% but less than or equal to 130%.
13. (canceled)
14. The control system of claim 1, operable to limit the rate of
increase of the applied drive torque to the prescribed maximum
value such that the maximum allowable rate of increase decreases as
a difference between an amount of drive torque currently applied to
the at least one driven wheel and the prescribed value of drive
torque decreases.
15. The control system of claim 1, configured to determine wheel
slip and to determine whether measured wheel slip is consistent
with a currently stored value of coefficient of surface friction
between the one or more wheels and the driving surface for the
amount of drive torque currently applied to the one or more wheels,
the control system being configured to update the currently stored
value of coefficient of surface friction in the event an
inconsistency is identified.
16. The control system of claim 1, operable to limit the amount of
drive torque that may be applied to one or more driven wheels of
the vehicle to the prescribed value so as to prevent the amount of
wheel slip from exceeding the prescribed value only when a
prescribed torque limit operational mode of the control system is
selected.
17. The control system of claim 16, operable to select the
prescribed torque limit operational mode when a corresponding
selection signal is received.
18. The control system of claim 1, operable to limit the amount of
drive torque applied to the one or more wheels in order to prevent
the amount of wheel slip from exceeding the prescribed value at
least in part by performing one or more selected from: causing a
reduction in an amount of drive torque transmitted from at least
one propulsion motor of the vehicle to the one or more wheels,
application of a brake to the one or more wheels, and reducing the
amount of drive torque generated by at least one propulsion motor
of the vehicle.
19.-20. (canceled)
21. A vehicle powertrain controller comprising the control system
of claim 1.
22. A vehicle comprising a control system that controls motion of
the vehicle over a surface, the control system being operable to
limit an amount of drive torque that may be applied to one or more
wheels of the vehicle that are arranged to be driven to a
prescribed torque value so as to prevent an amount of wheel slip
from exceeding a prescribed value of wheel slip, wherein the
prescribed value of wheel slip is determined in dependence at least
in part on vehicle speed, and wherein the control system is
operable to limit a rate of increase of applied drive torque to a
prescribed maximum value as the value of applied drive torque
increases toward the prescribed torque value.
23. The vehicle of claim 22, comprising a driver operable selector
for selecting the prescribed torque limit operational mode of the
control system.
24. The vehicle of claim 22, wherein the one or more wheels
comprise a pair of front wheels arranged to be driven and a pair of
rear wheels arranged to be driven.
25. The vehicle claim 22, wherein each of the one or more wheels
that is arranged to be driven is provided with a respective
propulsion motor.
26. The vehicle of claim 25, wherein each respective one or more
propulsion motors comprises an electric propulsion motor.
27. A method of controlling motion of a vehicle over a driving
surface via a control system, the method comprising: limiting an
amount of drive torque that may be applied to one or more driven
wheels of the vehicle to a prescribed torque value so as to prevent
an amount of wheel slip from exceeding a prescribed value;
determining the prescribed value of wheel slip in dependence at
least in part on vehicle speed; and limiting a rate of increase of
applied drive torque to a prescribed maximum value as the value of
applied drive torque increases toward the prescribed torque
value.
28-30. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention relates to motor vehicles and in
particular, but not exclusively, to a method of determining a speed
of a motor vehicle.
BACKGROUND
[0002] Traction control systems, also known as anti-slip regulation
(ASR) systems, are used on vehicles to prevent loss of traction of
the driven wheels so as to maintain control of the vehicle and to
prevent deterioration of the accelerating performance due to
longitudinal slip of the driving wheels. Traction control is
necessary, for example, when excessive accelerator input is applied
by the driver whilst driving on a road and the condition of the
road surface is such that it is unable to cope with the torque
applied.
[0003] Feedback control methods are typically employed in such
systems, whereby the slip of the driving wheels (wheel slip) is
monitored and, when it becomes excessive due to an excessive
driving torque (e.g. sudden application of the accelerator pedal),
appropriate action is taken to reduce the engine output and/or to
apply a braking force to the driving wheels. For example a traction
control system may signal an engine control unit (ECU) of the
vehicle to reduce engine torque by retarding or suppressing the
spark to one or more cylinders of the engine, reducing fuel supply
to one or more of the engine cylinders, closing the throttle, or,
in turbo-charged vehicles, actuating the boost control solenoid to
reduce boost and therefore engine power. Additionally, the wheel
brake to one or more of the wheels may be applied to control wheel
slip.
[0004] Traction control systems are typically implemented in a
vehicle as part of a stability control system (SCS) operable to
enhance stability of a vehicle by detecting and reducing skidding.
If a skid is detected whilst cornering, the stability control
system is arranged automatically to apply braking to individual
wheels to assist a driver in steering the vehicle in an intended
direction, for example around a corner.
[0005] It is desirable to provide a vehicle having improved
stability. This is particularly important in certain applications,
for example in vehicles having electric propulsion motors. This is
because electric propulsion motors are capable of delivering
relatively high values of drive torque and relatively high rates of
increase of drive torque.
[0006] It is also known to provide a control system for a motor
vehicle for controlling one or more vehicle subsystems. U.S. Pat.
No. 7,349,776, the content of which is hereby incorporated by
reference, discloses a vehicle control system comprising a
plurality of subsystem controllers including an engine management
system, a transmission controller, a steering controller, a brakes
controller and a suspension controller. The subsystem controllers
are each operable in a plurality of subsystem function modes. The
subsystem controllers are connected to a vehicle mode controller
which controls the subsystem controllers to assume a required
function mode so as to provide a number of driving modes for the
vehicle. Each of the driving modes corresponds to a particular
driving condition or set of driving conditions, and in each mode
each of the sub-systems is set to the function mode most
appropriate to those conditions. Such conditions are linked to
types of terrain over which the vehicle may be driven such as
grass/gravel/snow, mud and ruts, rock crawl, sand and a highway
mode known as `special programs off` (SPO). The vehicle mode
controller may be referred to as a Terrain Response (TR).RTM.
System or controller.
STATEMENT OF THE INVENTION
[0007] Embodiments of the invention may be understood with
reference to the appended claims.
[0008] Aspects of the present invention provide a vehicle and a
method.
[0009] A control system for controlling motion of a vehicle over a
surface, the control system being operable to limit an amount of
drive torque that may be applied to one or more wheels of the
vehicle that are arranged to be driven to a prescribed torque value
so as to prevent an amount of wheel slip from exceeding a
prescribed value, wherein the prescribed value of wheel slip is
determined in dependence at least in part on vehicle speed.
[0010] It is to be understood that because the prescribed value of
wheel slip is determined in dependence at least in part on vehicle
speed, the prescribed value of wheel torque is also dependent at
least in part on vehicle speed.
[0011] The prescribed value of wheel slip may be determined in
further dependence at least in part on a coefficient of friction
between the wheel and the surface.
[0012] Optionally, the prescribed value of wheel slip is determined
in further dependence at least in part on a value of surface drag
experienced by the vehicle.
[0013] Optionally, the prescribed value of wheel slip is determined
in dependence on vehicle speed only.
[0014] Further optionally, the prescribed value of torque is
determined in dependence on vehicle speed and a coefficient of
friction between the wheel and the surface only.
[0015] Optionally, the control system is operable in one of a
plurality of respective different driving modes, wherein the
prescribed value of wheel slip is determined by reference to
vehicle speed and optionally one or more other parameters in
dependence on the driving mode in which the control system is
operating.
[0016] In some embodiments the system may be operable to determine
the prescribed value of wheel slip in dependence on selected
driving mode and vehicle speed only.
[0017] The control system may be operable in an on-highway driving
mode suitable for driving on road surfaces with relatively high
values of surface coefficient of friction, and that are relatively
hard and smooth. The system may be operable when in the on-highway
mode to determine the prescribed value of wheel slip by reference
to vehicle speed only. Other arrangements are also useful.
[0018] The system may be operable when in a grass/gravel/snow mode
to determine the prescribed value of wheel slip in further
dependence at least in part on a coefficient of friction between
the wheel and the surface. Other arrangements are also useful.
[0019] The prescribed value of wheel slip may be defined
substantially as that at which a maximum amount of tractive force
acting to cause movement of the vehicle may be achieved.
[0020] The prescribed value of wheel slip may correspond
substantially to a prescribed proportion of that at which a maximum
amount of tractive force causing movement of the vehicle may be
achieved.
[0021] The prescribed proportion may correspond to a value less
than that at which a maximum amount of tractive force causing
movement of the vehicle may be achieved.
[0022] The prescribed proportion may correspond substantially to
one selected from amongst from greater than or equal to 70% but
less than 90% and greater than or equal to 90% but less than
100%.
[0023] Alternatively, the prescribed proportion may correspond to a
value greater than that at which a maximum amount of tractive force
causing movement of the vehicle may be achieved.
[0024] The prescribed proportion may correspond substantially to
one selected from amongst greater than 100% but less than or equal
to 110%, greater than 110% but less than or equal to 120%, and
greater than 120% but less than or equal to 130%.
[0025] The control system may be operable to limit a rate of
increase of applied drive torque to a prescribed maximum value as
the value of applied drive torque increases toward the prescribed
(maximum allowable) value.
[0026] The control system may be operable to limit a rate of
increase of applied drive torque to a prescribed maximum value such
that the maximum allowable rate of increase decreases as a
difference between an amount of drive torque currently applied to
the at least one driven wheel and the prescribed (maximum
allowable) value of drive torque decreases.
[0027] The control system may be configured to determine wheel slip
and to determine whether measured wheel slip is consistent with a
currently stored value of coefficient of surface friction between
the wheel and the driving surface for the amount of drive torque
currently applied to the wheel, the control system being configured
to update the currently stored value of coefficient of surface
friction in the event an inconsistency is identified.
[0028] The control system may be operable to limit an amount of
drive torque that may be applied to one or more driven wheels of
the vehicle so as to prevent an amount of wheel slip from exceeding
a prescribed value only when a prescribed torque limit operational
mode of the control system is selected.
[0029] It is to be understood that the prescribed torque limit
operational mode may be the only torque limit mode the vehicle is
operable in. Alternatively it may be one of a plurality of torque
limit operational modes, being operational modes in which an amount
of drive torque applied to a driven wheel is limited to a
prescribed amount.
[0030] Optionally, the control system is operable to select the
prescribed torque limit operational mode when a corresponding
selection signal is received.
[0031] The selection signal may be generated in response to user
actuation of a torque limit mode selector. Alternatively or in
addition, the torque limit operational mode may be selected in
dependence on a selected driving mode. Thus the torque limit
operational mode may be selected automatically when the control
system is operating in a prescribed one or more driving modes. For
example, the torque limit operational mode may be selected
automatically when the control system is operating in a
grass/gravel/snow mode. Other arrangements are also useful.
[0032] The control system may be operable to limit the amount of
drive torque applied to the at least one of the plurality of driven
wheels in order to prevent the amount of wheel slip from exceeding
the prescribed value at least in part by causing a reduction in an
amount of drive torque transmitted from at least one propulsion
motor of the vehicle to the at least one wheel. The amount of
torque transmitted may be reduced by means of a clutch or any other
suitable means.
[0033] The control system may be operable to limit the amount of
drive torque applied to the at least one of the plurality of driven
wheels in order to prevent the amount of wheel slip from exceeding
the prescribed value at least in part by application of brake means
to the at least one of the plurality of driven wheels.
[0034] The control system may be operable to limit the amount of
drive torque applied to the at least one of the plurality of driven
wheels in order to prevent the amount of wheel slip from exceeding
the prescribed value at least in part by reducing an amount of
drive torque generated by at least one propulsion motor of the
vehicle.
[0035] In a further aspect of the invention for which protection is
sought there is provided a vehicle powertrain controller comprising
a control system according to a preceding aspect.
[0036] In a still further aspect of the invention for which
protection is sought there is provided a vehicle comprising a
control system according to a preceding aspect.
[0037] The vehicle may comprise a driver operable selector for
selecting the prescribed torque limit operational mode of the
control system.
[0038] The vehicle may comprise a pair of front wheels arranged to
be driven and a pair of rear wheels arranged to be driven.
[0039] Optionally, each of the one or more wheels that is arranged
to be driven is provided with a respective propulsion motor.
[0040] Optionally, each respective one or more propulsion motors
comprises an electric propulsion motor.
[0041] Alternatively or in addition the vehicle may comprise an
engine, optionally an internal combustion engine.
[0042] In a further aspect of the invention for which protection is
sought there is provided a method of controlling motion of a
vehicle over a driving surface by means of a control system, the
method comprising: [0043] limiting an amount of drive torque that
may be applied to one or more driven wheels of the vehicle so as to
prevent an amount of wheel slip from exceeding a prescribed
value.
[0044] In a still further aspect of the invention for which
protection is sought there is provided a control system for
controlling motion of a vehicle over a surface, the control system
being operable to limit an amount of drive torque that may be
applied to one or more driven wheels of the vehicle to a value
corresponding to that for which maximum tractive force may be
obtained.
[0045] In an aspect of the invention for which protection is sought
there is provided a control system for controlling motion of a
vehicle over a surface, the control system being operable to limit
an amount of drive torque that may be applied to one or more driven
wheels of the vehicle to a value greater than that for which
maximum tractive force may be obtained by a prescribed
proportion.
[0046] In one aspect of the invention for which protection is
sought there is provided control means for controlling motion of a
vehicle over a surface, the control means being operable to limit
an amount of drive torque that may be applied to one or more wheels
of the vehicle that are arranged to be driven so as to prevent an
amount of wheel slip from exceeding a prescribed value.
[0047] The term control means as used herein is intended to
include, without limitation, a controller such as a microprocessor
or the like.
[0048] Embodiments of the present invention have the advantage that
a risk that a powertrain applies drive torque to one or more driven
wheels in such a manner as to cause excessive slip, resulting in a
reduction in vehicle stability, may be reduced.
[0049] By drive torque is meant a net torque acting on a wheel,
applied in order to promote movement of the vehicle and not brake
torque, being torque applied to reduce movement of the vehicle. In
other words, a net positive torque promoting (and not opposing)
motion of the vehicle. In some embodiments the net torque may be a
net torque resulting from application of a (positive) drive torque
and a (negative) brake torque.
[0050] It is to be understood that, for some road wheel/tyre
specifications on a given surface (e.g. a given tyre type on dry
asphalt), there is a wheel speed offset above the actual vehicle
speed above which further wheel slip is not beneficial. This may
for example be because it does not lead to an increase in tractive
force of the wheel on the driving surface.
[0051] For many driving surfaces, the amount of tractive force
decreases when the amount of wheel slip exceeds a certain value. In
some embodiments the control means may be configured to limit wheel
slip to a value that is at or below the value above which
increasing wheel slip results in a decrease in tractive force for
the surface on which the vehicle is driving. In some alternative
embodiments the control means may be configured to limit wheel slip
to a value slightly above that value above which increasing wheel
slip results in a decrease in tractive force, as described
below.
[0052] In some embodiments the control means may be arranged to
limit the amount of drive torque that may be applied to the one or
more driven wheels such that the prescribed value of allowable slip
is not exceeded. It is to be understood that, provided the driver
demands an amount of drive torque that will not result in the
prescribed value of allowable slip being exceeded, the powertrain
may be permitted to apply drive torque to the one or more driven
wheels in such a manner that the value of driver demanded torque is
delivered.
[0053] Where two or more wheels are driven by the powertrain, the
control means may determine (where possible) an appropriate torque
distribution between the two or more wheels such that the
prescribed allowable value of slip of each wheel is not exceeded
whilst still delivering a total torque (being the sum of the torque
values applied to each of the two or more wheels) that is
substantially equal to the driver demanded torque.
[0054] In some embodiments the control means may take into account
possible differences between the value of prescribed allowable slip
of different respective driven wheels. For example in the case of a
four wheel drive vehicle the control means may take into account
the fact that rear wheels of the vehicle may have an increased
downward force thereon during periods of acceleration compared with
the front wheels. In such a case the rear wheels may have a
different maximum value of drive torque that may be applied thereto
in order to obtain slip corresponding to the maximum prescribed
value compared with the front wheels, when the vehicle is
accelerating. The control means may be arranged to take vehicle
inclination into account, for example responsive to a signal from
an inclination sensor or other means for determining
inclination.
[0055] Advantageously the control means may be configured to
determine a maximum amount of drive torque that may be applied to
the one or more wheels to prevent the amount of wheel slip from
exceeding the prescribed value.
[0056] The control means may be operable to determine the maximum
amount of drive torque according to at least one selected from
amongst a wheel speed and a coefficient of friction between a wheel
and the surface.
[0057] It is to be understood that the coefficient of friction may
correspond to an estimated value of coefficient of surface
friction.
[0058] The prescribed value of wheel slip may be defined
substantially as that at which a maximum amount of tractive force
acting to cause movement of the vehicle may be achieved.
[0059] It is to be understood that the maximum amount of
acceleration of the vehicle may be achieved when the amount of
force causing movement of the vehicle is maximised.
[0060] It is well known that, for a given vehicle wheel on a given
surface, as the amount of torque applied to the wheel so as to
cause longitudinal acceleration of the vehicle/wheel from rest is
increased, the net amount of torque causing longitudinal
acceleration of the vehicle/wheel typically increases substantially
linearly with applied torque, at least initially.
[0061] In some cases, such as in the case of a wheel having a
rubber tyre in contact with asphalt, above a certain amount of
wheel slip the increase in wheel slip required to obtain a given
increase in driving force causing longitudinal acceleration begins
to increase non-linearly until, above a maximum value, if the
amount of torque applied to the wheel continues to increase, the
amount of slip increases but the net driving force causing
longitudinal acceleration of the wheel begins to decrease.
[0062] It is desirable to optimise the amount of drive torque
applied to a wheel so as to prevent the amount of wheel slip from
becoming excessive, causing a deterioration in certain vehicle
performance characteristics such as achievable rate of acceleration
on a given surface and lateral stability.
[0063] In some embodiments, the control means is configured to
limit the amount of drive torque applied to the wheel to that
corresponding to that at which the maximum achievable driving (or
tractive) force is obtained.
[0064] The prescribed value of wheel slip may correspond
substantially to a prescribed proportion of that at which a maximum
amount of tractive force causing movement of the vehicle may be
achieved.
[0065] The prescribed proportion may correspond to a value less
than that at which a maximum amount of tractive force causing
movement of the vehicle may be achieved.
[0066] The prescribed proportion may correspond substantially to
one selected from amongst from greater than or equal to 70% but
less than 90% and greater than or equal to 90% but less than
100%.
[0067] It is to be understood that other values or ranges of values
are also useful.
[0068] Advantageously the prescribed proportion may correspond to a
value greater than that at which a maximum amount of tractive force
causing movement of the vehicle may be achieved.
[0069] The prescribed proportion may correspond substantially to
one selected from amongst greater than 100% but less than or equal
to 110%, greater than 110% but less than or equal to 120%, and
greater than 120% but less than or equal to 130%.
[0070] It is to be understood that other values or ranges of values
are also useful.
[0071] The control means may be operable to limit a rate of
increase of applied drive torque to a prescribed maximum value as
the value of applied drive torque increases toward the maximum
allowable value.
[0072] It is to be understood that in some embodiments the limit in
respect of maximum allowable rate of increase of drive torque may
be imposed only over a prescribed range of applied torque, for
example as the amount of applied drive torque reaches a value
within (say) 10% or 20% or 30% of the maximum allowable value of
drive torque. Other prescribed ranges are also useful.
[0073] The control means may be operable to limit a rate of
increase of applied drive torque to a prescribed maximum value such
that the maximum allowable rate of increase decreases as a
difference between an amount of drive torque currently applied to
the at least one driven wheel and the maximum allowable value of
drive torque decreases.
[0074] Thus the prescribed allowable value of rate of increase may
be a value imposed responsive to the difference between the amount
of drive torque currently applied to the at least one driven wheel
and the maximum allowable value, for example a value that is
proportional to the difference. Alternatively the prescribed value
may be a fixed value, the limit being applied when the difference
between the amount of drive torque currently applied to the at
least one driven wheel and the maximum allowable value falls below
a prescribed value. Other arrangements are also useful.
[0075] The control means may be configured to determine wheel slip
and to determine whether measured wheel slip is consistent with a
currently stored value of coefficient of surface friction between
the wheel and the driving surface for the amount of drive torque
currently applied to the wheel, the control means being configured
to update the currently stored value of coefficient of surface
friction in the event an inconsistency is identified.
[0076] Thus it is to be understood that the control means may
update the value of surface mu used by the control means in
response to a determination of wheel slip (for example in response
to a measurement of wheel speed and vehicle speed, or using a value
of wheel slip provided to the control means) and the amount of
torque applied to the wheel. This feature has the advantage that if
the value of surface mu changes, for example if a driving surface
changes from dry asphalt to ice or vice versa, and an unexpected
change in the amount of wheel slip for a given applied torque
occurs, the control means is able to update the value of surface mu
accordingly.
[0077] For example, in the event the vehicle moves from ice to dry
asphalt and the control means applies the value of surface mu for
the wheel on ice to determine expected wheel slip whilst the
vehicle is in fact operating on dry asphalt, the control means may
determine that the amount of wheel slip is less than that which is
expected and revise the value of surface mu employed accordingly.
Other arrangements are also useful.
[0078] Optionally the control means may be operable to limit an
amount of drive torque that may be applied to one or more driven
wheels of the vehicle so as to prevent an amount of wheel slip from
exceeding a prescribed value only when a prescribed operational
mode of the control means is selected.
[0079] Thus the functionality described may only be implemented
once a prescribed operational mode has been selected.
[0080] The control means may be operable to select the prescribed
operational mode when a corresponding selection signal is
received.
[0081] The prescribed mode may correspond for example to a launch
mode.
[0082] In a further aspect of the invention for which protection is
sought there is provided a vehicle powertrain controller comprising
control means according to the preceding aspect.
[0083] By combining the control means with a vehicle powertrain
controller, a speed of response of the powertrain to control
signals provided by the control means may be increased
substantially. In some embodiments the control means may otherwise
be required to transmit control signals to the powertrain
controller via a communications channel such as a controller area
network (CAN) bus, introducing a delay between issuance of a
communication by the control means and receipt of the communication
(and therefore any subsequent response) by the powertrain
controller. This delay may be substantially eliminated in some
embodiments by providing the control means as part of the
powertrain controller.
[0084] In a still further aspect of the invention for which
protection is sought there is provided a vehicle comprising control
means according to a preceding aspect.
[0085] Advantageously the vehicle may comprise a driver operable
selector for selecting the prescribed operational mode of the
control means.
[0086] The selector may for example be a launch mode selector as
described above.
[0087] The control means may be operable to limit the amount of
drive torque applied to the at least one of the plurality of driven
wheels in order to prevent the amount of wheel slip from exceeding
the prescribed value by reducing an amount of drive torque
transmitted from the at least one propulsion motor to the at least
one wheel.
[0088] In some embodiments, disconnection of clutch means or
slipping of clutch means may be employed to reduce the amount of
drive torque transmitted.
[0089] Alternatively or in addition the control means may be
operable to limit the amount of drive torque applied to the at
least one of the plurality of driven wheels in order to prevent the
amount of wheel slip from exceeding the prescribed value by
application of brake means to the at least one of the plurality of
driven wheels.
[0090] Further alternatively or in addition the control means may
be operable to limit the amount of drive torque applied to the at
least one of the plurality of driven wheels in order to prevent the
amount of wheel slip from exceeding the prescribed value by
reducing an amount of drive torque generated by the at least one
propulsion motor of the vehicle.
[0091] The vehicle may comprise a pair of front wheels arranged to
be driven and a pair of rear wheels arranged to be driven.
[0092] Optionally each of the one or more wheels that is arranged
to be driven is provided with a respective propulsion motor.
[0093] Optionally each respective one or more propulsion motors
comprises an electric propulsion motor.
[0094] It is to be understood that since electric propulsion motors
can develop relatively high torque values, embodiments of the
invention may be particularly suitable for applications in which
two, three four or more wheels of a vehicle are each provided with
a respective electric propulsion motor.
[0095] In some embodiments the control means may be configured to
perform a vehicle speed check in order to verify that a vehicle
speed employed by the control means corresponds to actual vehicle
speed. The control means may perform the speed check by reducing an
amount of torque applied to one or more wheels that are arranged to
be driven and measuring wheel speed following the torque reduction.
In some embodiments the torque may be reduced until wheel speed no
longer reduces with reducing amount of torque. In some embodiments
the torque may be reduced until a rate of reduction in wheel speed
with reducing amount of torque falls below a prescribed value.
Other arrangements are also useful. Aspects of this control
methodology which may be used to verify vehicle speed are disclosed
in UK patent application no. GB1210273.7 which is hereby
incorporated herein by reference
[0096] In another aspect of the invention for which protection is
sought there is provided a method of controlling motion of a
vehicle over a driving surface by means of control means, the
method comprising: [0097] limiting an amount of drive torque that
may be applied to one or more driven wheels of the vehicle so as to
prevent an amount of wheel slip from exceeding a prescribed
value.
[0098] In an aspect of the invention for which protection is sought
there is provided control means for controlling motion of a vehicle
over a surface, the control means being operable to limit an amount
of drive torque that may be applied to one or more driven wheels of
the vehicle to a value corresponding to that for which maximum
tractive force may be obtained.
[0099] In a further aspect of the invention for which protection is
sought there is provided control means for controlling motion of a
vehicle over a surface, the control means being operable to limit
an amount of drive torque that may be applied to one or more driven
wheels of the vehicle to a value greater than that for which
maximum tractive force may be obtained by a prescribed
proportion.
[0100] The prescribed proportion may for example be one selected
from amongst 10%, 20%, 30%, 40% or 50% of the maximum value. Other
values are also useful.
[0101] This feature has the advantage that the control means may be
arranged to allow the amount of wheel slip to increase to a value
sufficient to cause a drop in tractive force, allowing the control
means to verify more readily (in some arrangements) whether a
current value of coefficient of surface friction employed by the
control means is substantially correct. As noted above, in some
embodiments the control means may monitor an amount of wheel slip
as a function of applied drive torque and update the value of
coefficient of friction between the wheel and the surface that is
used by the control means if the control determines that a
discrepancy exists in the observed relationship between wheel slip
and applied drive torque. In some embodiments the control means may
correlate one or more other parameters such as vehicle acceleration
(for example as determined by an accelerometer) with wheel slip or
applied drive torque in order to verify the current value of
coefficient of friction employed by the control means.
[0102] In another aspect of the invention for which protection is
sought there is provided a motor vehicle having at least one
propulsion motor for generating drive torque to drive the vehicle
and control means for controlling motion of a vehicle over a
driving surface, the control means being operable to: [0103]
estimate a maximum value of allowable slip of one or more driven
wheels; and [0104] limit an amount of drive torque applied to the
one or more driven wheels so as to prevent the amount of wheel slip
from exceeding the maximum allowable value.
[0105] Within the scope of this application it is envisaged that
the various aspects, embodiments, examples and alternatives set out
in the preceding paragraphs, in the claims and/or in the following
description and drawings, and in particular the individual features
thereof, may be taken independently or in any combination. Features
described with reference to one embodiment are applicable to all
embodiments, unless there is incompatibility of features.
BRIEF DESCRIPTION OF THE DRAWINGS
[0106] Embodiments of the invention will now be described, by way
of example only, with reference to the accompanying figures in
which:
[0107] FIG. 1 is a schematic illustration of a vehicle according to
an embodiment of the present invention;
[0108] FIG. 2 is a flow diagram of a method of controlling a
vehicle; and
[0109] FIG. 3 is a plot of `torque to ground`, which is
proportional to `force to ground` being the useful longitudinal
force causing vehicle acceleration due to application of torque to
a wheel, as a function of wheel slip (being the ratio of wheel
speed to ground speed).
DETAILED DESCRIPTION
[0110] FIG. 1 shows a four wheel drive motor vehicle 100 according
to an embodiment of the present invention. The vehicle has four
wheels 111, 112, 113, 114 each arranged to be driven by a
respective electric propulsion motor 111M, 112M, 113M, 114M. The
vehicle has a controller 140 configured to control an amount of
torque delivered to each wheel 111, 112, 113, 114 by each
respective motor individually. A battery 150 supplies power to
drive the motors 111M, 112M, 113M, 114M.
[0111] The controller 140 is operable to determine an amount of
drive torque to be developed at the wheels 111, 112, 113, 114 to
drive the vehicle as demanded by a driver of the vehicle responsive
to a position of an accelerator pedal 181. An anti-lock braking
system (ABS) controller 161 is configured to control braking of the
vehicle 100 responsive to a position of a driver brake pedal
183.
[0112] In response to driver demand for torque, the controller 140
is configured to control the amount of torque delivered by each of
the motors 111M, 112M, 113M, 114M to its respective wheel. The
controller 140 is operable to implement a feed forward control
methodology in order to reduce a risk that wheel slip occurs due to
the application of excessive torque to the wheels 111, 112, 113,
114. It is to be understood that on some driving surfaces it may be
possible for a driver to demand an amount of drive torque that
would result in wheel spin if the controller 140 were to allow the
full amount of torque demanded by the driver to be applied.
[0113] FIG. 2 is a high level schematic illustration of a sequence
of steps performed by the controller 140 during vehicle operations
in implementing the feed forward control methodology. It is to be
understood that the steps may be implemented in conjunction with
one or more other steps, before, between or after the steps
illustrated.
[0114] In some embodiments the steps are performed whenever the
vehicle is being driven. In some embodiments the steps may be
performed only if a particular vehicle mode such as a launch
control mode, sport mode and/or one or more other modes are
selected.
[0115] At step S101 the controller 140 determines a current value
of driver demanded torque Tq_dd based on accelerator pedal
position. In some embodiments one or more other parameters may be
employed by the controller 140 in addition to pedal position in
order to determine the value of Tq_dd, such as engine speed in some
embodiments having an engine. In some embodiments the controller
140 may be provided with a value of Tq_dd that has been determined
by a different controller, for example by an engine controller in
the case of a vehicle having an engine.
[0116] It is to be understood that a torque demand signal from a
speed control system such as a cruise control system or off-road
speed control system may be employed instead, in some embodiments,
if the speed control system is active. The off-road speed control
system may be a system operable to control vehicle speed in
accordance with a target speed despite the occurrence of wheel
slip. The off-road speed control system may be referred to as a
low-speed speed control system in some embodiments. The off-road
speed control system may be operable only below a certain speed,
such as a speed in the range from 30 km/h to 60 km/h, optionally
around 50 km/h. Other arrangements are also useful.
[0117] At step S103 the controller 140 determines an estimated
value of maximum allowable slip, slip_max, that is to be allowed by
a wheel for the current driving surface. The determination is made
using a current value of coefficient of friction between each wheel
and the surface over which the vehicle 100 is moving (referred to
herein as `surface mu` or .mu.), and a current value of vehicle
speed.
[0118] It is to be understood that the controller 140 may be
arranged to determine the current value of surface mu based on an
assumption that the value is 1 (corresponding to dry asphalt)
unless the controller 140 determines that a lower value is more
appropriate. The controller 140 may for example determine that a
lower value is appropriate responsive to detection of wheel slip
exceeding that which would be expected on such a surface for given
values of one or more of vehicle longitudinal acceleration, lateral
acceleration, wheel speed and net torque applied to a wheel.
[0119] FIG. 3 shows a plot of torque to ground as a function of
wheel slip for given values of surface mu (.mu.) and vehicle speed
(s). The amount of torque applied to the wheel (as opposed to the
torque to ground, being the useful torque acting to cause
longitudinal acceleration) increases with wheel slip as indicated
in the figure. It can be seen that as the amount of wheel slip
increases the amount of useful torque to ground increases until a
maximum value Tq_max is reached, corresponding to a peak driving
force on the vehicle. It can be seen that if the amount of torque
applied to the wheel increases further, the amount of wheel slip
increases but the amount of driving force acting on the vehicle
decreases. It is to be understood that the precise values of wheel
torque as a function of wheel slip depend on the coefficient of
friction between the wheel and the driving surface (which depends
for example on the tyre and driving surface) and vehicle speed.
[0120] As noted above, at step S103 the controller 140 identifies
the value of slip_max, which is a value corresponding to Tq_max,
according to the current value of surface mu and vehicle speed. In
some embodiments the value of slip_max may be determined for
example by reference to one or more look-up tables.
[0121] At step S105 the controller 140 determines whether the
current value of Tq_dd is sufficiently high to cause excessive
wheel slip. In the present embodiment the controller 140 determines
how much torque should be applied to each wheel in order to deliver
a total torque substantially equal to Tq_dd. Furthermore, in the
present embodiment excessive wheel slip is defined as wheel slip in
excess of slip_max. The controller 140 can therefore determine the
corresponding value of torque to ground, Tq_max.
[0122] The controller 140 then determines, for each wheel, whether
the amount of torque that would be required to be applied to that
wheel in order to meet driver demand would exceed the current value
of Tq_max for that wheel. In some embodiments, the controller 140
may be arranged to determine the value of Tq_max from a look-up
table, either using a value of slip_max after determining the value
of slip_max, or directly, without determining the value of slip_max
as an intermediate step.
[0123] It is to be understood that because the value of Tq_max for
a given wheel may depend on a weight applied to the wheel, the
value of Tq_max may be different for different wheels. In the case
of a vehicle that is accelerating, it may for example be expected
for some vehicles that a weight on rear wheels of the vehicle will
be higher than front wheels, and therefore the value of Tq_max may
also be higher for the rear wheels compared with the front wheels.
In some embodiments the controller 140 is configured to take such
variations into account in determining whether Tq_max will be
exceeded for one or more wheels. In some embodiments, the
controller 140 may estimate an increase in weight borne by rear
wheels of the vehicle when the vehicle is accelerating relative to
the weight borne when the vehicle is not accelerating. The
controller 140 may in addition or instead estimate a decrease in
weight borne by front wheels of the vehicle when the vehicle is
accelerating relative to the weight borne when the vehicle is not
accelerating.
[0124] If the current value of Tq_dd is not sufficiently high to
cause excessive wheel slip of any of the driven wheels 111, 112,
113, 114 (i.e. the value of torque to be applied to a given wheel
will not exceed the value of Tq_max for that wheel) the controller
140 allows the current value of Tq_dd to be delivered via the
wheels 111, 112, 113, 114 and continues at step S101.
[0125] In some embodiments, the controller 140 may be arranged to
distribute torque amongst the wheels 111, 112, 113, 114 in an
optimum manner so as to reduce a risk that the amount of torque
applied to any one wheel exceeds the value of Tq_max for that
wheel.
[0126] If at step S105 the controller 140 determines that the
current value of Tq_dd is sufficiently high to cause excessive
wheel slip if the controller 140 were to attempt to deliver
corresponding amounts of torque to each wheel, the controller 140
is configured to limit the amount of torque delivered to each wheel
111, 112, 113, 114 to a value sufficient to allow each wheel to
experience slip up to a value slip_max for that wheel.
[0127] At step S109 the controller 140 determines whether actual
amounts of slip experienced by each wheel 111, 112, 113, 114 as
measured by the controller 140 are consistent with the current
estimated value of surface mu. If the amounts are consistent, the
controller continues at step S101.
[0128] If the amounts are not consistent, then at step S111 the
controller 140 calculates a revised estimated value of surface mu.
The controller then continues at step S101.
[0129] In some embodiments, instead of limiting an amount of slip
of each wheel to the value slip_max (by limiting the amount of
torque applied to a wheel to a value corresponding to Tq_max), the
controller 140 may be configured to limit the amount of slip to a
value that is less than slip_max, or in some other embodiments to a
value that is greater than slip_max.
[0130] For example, the controller 140 may be arranged to limit the
amount of wheel slip to a value that is 10% greater than slip_max.
This feature has the advantage that the controller 140 is able more
quickly to identify a situation in which the actual value of
coefficient of friction between the driving surface and wheel is
greater than the currently estimated value.
[0131] In the present embodiment the controller 140 is configured
repeatedly to check whether the current value of wheel slip
corresponds to the value expected for the currently estimated value
of surface mu and vehicle speed. If the amount of wheel slip is
limited to value greater than slip_max, then if when the amount of
torque applied to a wheel exceeds the estimated value of Tq_max but
the amount of wheel slip has not yet reached slip_max, the
controller 140 may determine that the actual or prevailing value of
surface mu is greater than the currently estimated value. The
controller 140 may therefore take steps to remedy this discrepancy.
In some embodiments the controller 140 may continue to allow
greater values of torque to be applied to a wheel (where this is
desirable in order to meet driver demand) and to check the
corresponding value of wheel slip to see whether the value has
reached slip_max.
[0132] In some embodiments, in the event a discrepancy in the value
of surface mu is identified the controller 140 may be arranged to
increment or decrement the value of surface mu, depending on
whether the value of surface mu employed appears to be too low or
too high, by a prescribed amount. The prescribed amount may for
example be 0.1, 0.05, 0.01 or any other suitable value. The
controller 140 may maintain this revised value as the value
employed thereby until a determination is again made that the
currently estimated value is too low (or too high).
[0133] In the present embodiment the controller 140 is arranged to
assume that the amount of torque applied to a wheel by the vehicle
powertrain corresponds to the amount of reacted torque for values
of slip up to slip_max. Other arrangements are also useful.
[0134] It is to be understood that embodiments of the present
invention implement a feedforward control methodology in order to
reduce a risk that a vehicle 100 suffers excessive wheel slip. As
noted above excessive wheel slip may be experienced for example
during periods of harsh acceleration or whilst operating on a
surface having a reduced value of surface mu. It is found that
vehicles implementing such a control methodology exhibit
significantly enhanced performance characteristics including a
substantial reduction in wheel slip during acceleration from
rest.
[0135] Embodiments of the present invention are also suitable for
implementation in vehicles operable in different driving modes such
as terrain response modes as described above. A vehicle controller
such as controller 140 or other controller of the vehicle may be
configured to implement a known Terrain Response (TR).RTM. System
of the kind described above in which the controller 140 controls
settings of one or more vehicle systems or sub-systems such as a
powertrain controller, the ABS controller 161, a suspension system
controller (where present) and/or any other vehicle subsystem in
dependence on a selected driving mode.
[0136] The driving mode may be selected by a user by means of a
driving mode selector switch or other control. The driving modes
may also be referred to as terrain modes, terrain response modes,
or control modes.
[0137] In some embodiments, at least four driving modes may be
provided: an `on-highway` driving mode suitable for driving on a
relatively hard, smooth driving surface where a relatively high
surface coefficient of friction exists between the driving surface
and wheels of the vehicle; a `sand` driving mode suitable for
driving over sandy terrain; a `grass, gravel or snow` driving mode
suitable for driving over grass, gravel or snow; a `rock crawl`
driving mode suitable for driving slowly over a rocky surface; and
a `mud and ruts` driving mode suitable for driving in muddy and/or
rutted terrain. Other driving modes may be provided in addition or
instead.
[0138] In some embodiments the vehicle may be operable in an
automatic driving mode selection condition in which the vehicle is
configured to determine automatically the most appropriate driving
mode for the prevailing terrain. The vehicle may be provided with
sensors (not shown) for sensing a variety of different parameters
associated with vehicle motion and status. These may be inertial
systems, including for example gyros and/or accelerometers that may
form part of an occupant restraint system or any other sub-system
requiring data indicative of vehicle body movement. The signals
from the sensors may be used to provide, or used to calculate, a
plurality of driving condition indicators (also referred to as
terrain indicators) which are indicative of the nature of the
terrain conditions over which the vehicle is travelling.
[0139] The sensors may also include, but may not be limited to,
sensors providing continuous sensor outputs to the VCU 10,
including wheel speed sensors, as mentioned previously with respect
to FIG. 1, an ambient temperature sensor, an atmospheric pressure
sensor, tyre pressure sensors, wheel articulation sensors,
gyroscopic sensors to detect vehicular yaw, roll and pitch angle
and rate, a vehicle speed sensor, a longitudinal acceleration
sensor, an engine torque sensor (or engine torque estimator), a
steering angle sensor, a steering wheel speed sensor, a gradient
sensor (or gradient estimator), a lateral acceleration sensor which
may be part of a stability control system (SCS), a brake pedal
position sensor, a brake pressure sensor, an accelerator pedal
position sensor, longitudinal, lateral and vertical motion sensors,
and water detection sensors forming part of a vehicle wading
assistance system.
[0140] The controller 140 may also receive a signal from a steering
controller. The steering controller may be in the form of an
electronic power assisted steering unit (ePAS unit). The steering
controller may be arranged to provide a signal to the controller
140 indicative of the steering force being applied to steerable
road wheels 111, 112 of the vehicle. This force may correspond to
that applied by a user to the steering wheel 171 in combination
with steering force generated by the ePAS unit.
[0141] The controller 140 may be configured to evaluate the various
sensor inputs to determine the probability that each of the
plurality of different driving modes (or control modes) for the
vehicle subsystems is appropriate.
[0142] If the user has selected operation of the vehicle in the
automatic driving mode selection condition, the controller 140 may
then select the most appropriate one of the control modes and be
configured automatically to control the subsystems according to the
selected mode. This aspect of the invention is described in further
detail in our co-pending patent application nos. GB1111288.5,
GB1211910.3 and GB1202427.9, the contents of each of which is
incorporated herein by reference.
[0143] The controller 140 may be operable to determine a maximum
allowable value of drive torque that may be applied to one or more
wheels of the vehicle that are arranged to be driven so as to
prevent an amount of wheel slip from exceeding a prescribed value,
the prescribed value being determined in dependence at least in
part on vehicle speed and the driving mode in which the vehicle 100
is driving.
[0144] For example, in some embodiments if the vehicle is driving
in a mode optimised for highway driving (i.e. on relatively smooth
surfaces having a relatively high coefficient of friction with the
vehicle wheels 111, 112, 113, 114) the maximum allowable value of
drive torque may tend towards a higher value in dependence on
vehicle speed (the value tending to increase with speed).
[0145] In contrast, in some embodiments if the vehicle is driving
in a mode optimised for driving on grass/gravel/snow (i.e.
relatively smooth surfaces having a relatively low coefficient of
friction with the vehicle wheels 111, 112, 113, 114) the maximum
allowable value of drive torque may be set to a lower (and in some
alternative embodiments to a higher) value than that which would be
employed for operation in a highway driving mode. Again, the value
of maximum allowable drive torque may tend to increase with vehicle
speed.
[0146] In some embodiments, the value of maximum allowable drive
torque may be calculated in dependence on one or more of a
plurality of vehicle parameters, the identity of the parameters
employed depending on the driving mode. For example, if the vehicle
is operating in the on-highway driving mode, the maximum allowable
value of drive torque may be determined in dependence on vehicle
speed only. In contrast, if the vehicle is operating in the
grass/gravel/snow driving mode, the maximum allowable value of
drive torque may be determined in dependence on vehicle speed and
coefficient of friction between the driving surface and vehicle
wheels. The controller 140 may be provided with real time updates
in respect of the instant value of surface coefficient of friction.
The real time updates may be provided by means of a controller area
network (CAN) bus or by any other suitable means.
[0147] It is to be understood that in the case of operation in the
on-highway driving mode, the controller 140 may assume that the
value of surface coefficient of friction is substantially unity, or
close to unity. Similarly, a substantially fixed value of surface
coefficient of friction may be employed for one or more other modes
in addition or instead.
[0148] In the present embodiment, the controller is configured to
decrease the maximum allowable value of slip with decreasing
surface coefficient of friction.
[0149] In some embodiments the controller 140 may be configured to
calculate the prescribed value of slip in further dependence on a
parameter indicative of an amount of surface drag experienced by
the vehicle. It is to be understood that on certain relatively soft
terrain such as grass, gravel, snow and sand, surface drag is found
to be higher than that experienced on relatively hard, smooth
surfaces such as asphalt. The controller 140 may be configured such
that as surface drag increases the maximum allowable value of wheel
slip is increased. The maximum allowable value may increase with
increasing surface drag in some embodiments. In some embodiments
the maximum allowable value of wheel slip may be higher for travel
in sand mode compared with the grass/gravel/snow mode.
[0150] As noted above, embodiments of the present invention have
the advantage that significantly enhanced performance
characteristics may be enjoyed when operating in off-road and
on-road (or on-highway) terrain conditions. Some embodiments of the
invention permit a substantial reduction in wheel slip experienced
by a vehicle during acceleration from rest over a variety of
different types of terrain, compared with acceleration from rest on
the same terrain by known vehicles.
[0151] Embodiments of the present invention may be understood by
reference to the following numbered paragraphs: [0152] 1. A control
system for controlling motion of a vehicle over a surface, the
control system being operable to limit an amount of drive torque
that may be applied to one or more wheels of the vehicle that are
arranged to be driven to a prescribed torque value so as to prevent
an amount of wheel slip from exceeding a prescribed value, wherein
the prescribed value of wheel slip is determined in dependence at
least in part on vehicle speed. [0153] 2. A control system
according to paragraph 1 wherein the prescribed value of wheel slip
is determined in further dependence at least in part on a
coefficient of friction between the wheel and the surface. [0154]
3. A control system according to paragraph 1 wherein the prescribed
value of wheel slip is determined in further dependence at least in
part on a value of surface drag experienced by the vehicle. [0155]
4. A control system according to paragraph 1 wherein the prescribed
value of wheel slip is determined in dependence on vehicle speed
only. [0156] 5. A control system according to paragraph 2 wherein
the prescribed value of torque is determined in dependence on
vehicle speed and a coefficient of friction between the wheel and
the surface only. [0157] 6. A control system according to paragraph
1 wherein the control system is operable in one of a plurality of
respective different driving modes, wherein the prescribed value of
wheel slip is determined by reference to vehicle speed and
optionally one or more other parameters in dependence on the
driving mode in which the control system is operating. [0158] 7. A
control system as described in paragraph 1 wherein the prescribed
value of wheel slip is defined substantially as that at which a
maximum amount of tractive force acting to cause movement of the
vehicle may be achieved. [0159] 8. A control system as described in
paragraph 1 wherein the prescribed value of wheel slip corresponds
substantially to a prescribed proportion of that at which a maximum
amount of tractive force causing movement of the vehicle may be
achieved.
[0160] 9. A control system as described in paragraph 8 wherein the
prescribed proportion corresponds to a value less than that at
which a maximum amount of tractive force causing movement of the
vehicle may be achieved. [0161] 10. A control system as described
in paragraph 8 wherein the prescribed proportion corresponds
substantially to one selected from amongst from greater than or
equal to 70% but less than 90% and greater than or equal to 90% but
less than 100%. [0162] 11. A control system as described in
paragraph 8 wherein the prescribed proportion corresponds to a
value greater than that at which a maximum amount of tractive force
causing movement of the vehicle may be achieved. [0163] 12. A
control system as described in paragraph 11 wherein the prescribed
proportion corresponds substantially to one selected from amongst
greater than 100% but less than or equal to 110%, greater than 110%
but less than or equal to 120%, and greater than 120% but less than
or equal to 130%. [0164] 13. A control system as described in
paragraph 1 operable to limit a rate of increase of applied drive
torque to a prescribed maximum value as the value of applied drive
torque increases toward the prescribed value. [0165] 14. A control
system as described in paragraph 13 operable to limit a rate of
increase of applied drive torque to a prescribed maximum value such
that the maximum allowable rate of increase decreases as a
difference between an amount of drive torque currently applied to
the at least one driven wheel and the prescribed value of drive
torque decreases. [0166] 15. A control system as described in
paragraph 1 configured to determine wheel slip and to determine
whether measured wheel slip is consistent with a currently stored
value of coefficient of surface friction between the wheel and the
driving surface for the amount of drive torque currently applied to
the wheel, the control system being configured to update the
currently stored value of coefficient of surface friction in the
event an inconsistency is identified. [0167] 16. A control system
as described in paragraph 1 operable to limit the amount of drive
torque that may be applied to one or more driven wheels of the
vehicle to the prescribed value so as to prevent the amount of
wheel slip from exceeding the prescribed value only when a
prescribed torque limit operational mode of the control system is
selected. [0168] 17. A control system as described in paragraph 16
operable to select the prescribed torque limit operational mode
when a corresponding selection signal is received. [0169] 18. A
control system according to paragraph 1 operable to limit the
amount of drive torque applied to the at least one of the plurality
of driven wheels in order to prevent the amount of wheel slip from
exceeding the prescribed value at least in part by causing a
reduction in an amount of drive torque transmitted from at least
one propulsion motor of the vehicle to the at least one wheel.
[0170] 19. A control system according to paragraph 1 operable to
limit the amount of drive torque applied to the at least one of the
plurality of driven wheels in order to prevent the amount of wheel
slip from exceeding the prescribed value at least in part by
application of a brake to the at least one of the plurality of
driven wheels. [0171] 20. A control system as described in
paragraph 1 operable to limit the amount of drive torque applied to
the at least one of the plurality of driven wheels in order to
prevent the amount of wheel slip from exceeding the prescribed
value at least in part by reducing the amount of drive torque
generated by at least one propulsion motor of the vehicle. [0172]
21. A vehicle powertrain controller comprising a control system as
described in paragraph 1. [0173] 22. A vehicle comprising a control
system as described in paragraph 1. [0174] 23. A vehicle as
described in paragraph 22 comprising a driver operable selector for
selecting the prescribed torque limit operational mode of the
control system. [0175] 24. A vehicle as described in paragraph 22
comprising a pair of front wheels arranged to be driven and a pair
of rear wheels arranged to be driven. [0176] 25. A vehicle as
described in paragraph 22 wherein each of the one or more wheels
that is arranged to be driven is provided with a respective
propulsion motor. [0177] 26. A vehicle as described in paragraph 25
wherein each respective one or more propulsion motors comprises an
electric propulsion motor. [0178] 27. A method of controlling
motion of a vehicle over a driving surface by means of a control
system, the method comprising: [0179] limiting an amount of drive
torque that may be applied to one or more driven wheels of the
vehicle to a prescribed torque value so as to prevent an amount of
wheel slip from exceeding a prescribed value; and [0180]
determining the prescribed value of wheel slip in dependence at
least in part on vehicle speed. [0181] 28. A control system for
controlling motion of a vehicle over a surface, the control system
being operable to limit an amount of drive torque that may be
applied to one or more driven wheels of the vehicle to a value
corresponding to that for which maximum tractive force may be
obtained. [0182] 29. A control system for controlling motion of a
vehicle over a surface, the control system being operable to limit
an amount of drive torque that may be applied to one or more driven
wheels of the vehicle to a value greater than that for which
maximum tractive force may be obtained by a prescribed
proportion.
[0183] Throughout the description and claims of this specification,
the words "comprise" and "contain" and variations of the words, for
example "comprising" and "comprises", means "including but not
limited to", and is not intended to (and does not) exclude other
moieties, additives, components, integers or steps.
[0184] Throughout the description and claims of this specification,
the singular encompasses the plural unless the context otherwise
requires. In particular, where the indefinite article is used, the
specification is to be understood as contemplating plurality as
well as singularity, unless the context requires otherwise.
[0185] Features, integers, characteristics, compounds, chemical
moieties or groups described in conjunction with a particular
aspect, embodiment or example of the invention are to be understood
to be applicable to any other aspect, embodiment or example
described herein unless incompatible therewith.
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